1. Field of the Invention
The present invention relates to a contact-sensing device including a capacitance detecting sensor as a user input device, a mobile information terminal including the contact-sensing device, and a capacitance calibration program and method.
2. Description of the Related Art
In recent years, many mobile devices having touch keys, such as mobile phones, have been commercialized. The touch keys installed in such mobile information terminals are often so-called capacitance touch sensors. Touch keys composed of capacitance touch sensors are capable of detecting a pushing action by a user by, for example, monitoring the change in capacitance when the user's finger comes into contact.
A capacitance touch sensor has a characteristic in that the capacitance varies greatly in response to a temperature change.
Causes of changing the temperature of a touch sensor may include an environmental change (temperature change) and a temperature increase in the terminal containing the touch sensor. A temperature change due to the environment occurs over a relatively long period of time. A temperature increase in the terminal, however, often occurs suddenly in a short period of time. A temperature increase in the terminal is caused by a temperature increase in the CPU and/or battery due to, for example, running a plurality of applications at the same time and/or use under high load, such as carrying out processing for an application for a long period of time.
A change in the capacitance of the touch sensor often occurs when the user slightly contacts a touch key other than the desired touch key with their finger. When a plurality of touch keys are provided in a limit space on a small casing, such as a mobile phone, each touch key is arranged adjacent to each other. Therefore, a capacitance change due to contacting an unexpected touch key easily occurs. When a finger is in contact with a touch sensor, the capacitance of the touch sensor changed due to the heat of the finger.
As described above, the capacitance of the touch keys having capacitance touch sensors easily varies due to a temperature change and/or unintentional contact.
For example, when a user long-pushes a desired touch key, their finger may unintentionally contact the adjacent touch key, causing the capacitance of the adjacent touch key to increase. In such a state, the long-push of the desired touch key causes application processing to be carried out for a long period of time and thus causes an increases in load. As a result, the terminal temperature increases, causing a change in the capacitance of each touch sensor. When this occurs, the adjacent touch key is highly likely to be mistakenly turned on.
A mobile phone according to the related art sets a time limit for a long-push of a desired touch key in order to prevent such malfunctions. In other words, when the long-push state exceeds the time limit, the mobile phone nullifies the touch keys once, and calibrates the touch keys. Calibration of the touch keys is carried out, for example, through a process of resetting the reference value of the capacitance.
As touch screen calibration system and method, Japanese Translation of PCT International Application, Publication No. 2005-512197 describes a touch screen calibration system including a touch screen having a plurality of terminals, a control circuit configured to apply at least one signal to the terminals and detects the effect of touching the touch screen on the signal, and a microprocessor configured to calculate mismeasurement by applying calibration impedance and acquiring a corrected touch position from a measured touch position by applying the mismeasurement in response to the touching.